The Lewis structure for SeO2, known as selenium dioxide, is one atom of selenium doubly bonded to a molecule of oxygen on each side, resulting in a total of two atoms of oxygen. Selenium's electrons are a completed pair, and the molecule does not have any lone pairs of electrons. The oxygen molecules bonded to selenium each have two lone pairs of electrons.
Selenium dioxide is a colorless solid that consists of a one-dimensional polymeric chain with alternating atoms of selenium and oxygen. It is an acidic oxide and dissolves to form selenous acid. Selenium compounds are poisonous and contain a strong odor. Because of this, working with the compound requires appropriate fume hoods and precautions.
It is used in the Riley oxidation. The reaction is carried out using a catalytic amount of SeO2 with the addition of an oxidizing agent like t-butyl hydroperoxide. This re-oxidizes the selenium (II) compounds after each cycle of the reaction and eliminates the need to remove large amounts of toxic selenium compounds. In addition, it ensures there is no further oxidation to conjugated carbonyl compounds. There are several solvents that can be used with the compound, including acetic acid, which results in the formation of acetate esters. This stops the Riley reaction at the allylic alcohol step.